Ezine

Published: Dec 19, 2016

Author: Jon Evans

Channels: Sample Preparation

Diamonds are forever

Crude oil is dark, viscous and highly complex, made up of a whole range of different compounds, which can make extracting specific compounds from it for analysis a long, difficult and often environmentally-unfriendly process. Now, though, a team of Chinese scientists has come up with a novel method for extracting specific analytes from crude oil that takes only 10 minutes and requires just 20μL of organic solvent. Originally developed for extracting diamondoids, the scientists are confident their method could be adapted both to extract other compounds from crude oil and also to work with other complex samples.

As their name suggests, diamondoids are essentially nanoscale diamonds. They are made from the carbon molecule adamantane (C10H16), which is the smallest molecular unit in the diamond crystal lattice. Because diamondoids can consist of up to 11 adamantane molecules – for example, a diamondoid made up of two adamantane molecules is known as diamantane – and because adamantane can exist as various structural variants, there is a large family of different diamondoids.

This family is naturally found in crude oil, where their distribution and abundance depends on the precise geological and chemical history of the oil. This means that researchers can discover a great deal about a crude oil sample, including its age and origin, by analyzing its complement of diamondoids. First off, though, they need to extract the diamondoids from the oil and while there are several methods for doing this, such as column chromatography, they tend to be complicated, time-consuming and require lots of toxic organic solvents.

Gas purge

So the team of Chinese scientists, led by Shukui Zhu at the China University of Geoscience in Wuhan, decided to try developing an extraction method that was faster, more efficient and more sustainable. The end result was a homemade device for conducting a novel method the scientists termed gas purge microsyringe extraction (GP-MSE). The device consists of a hollow silica tube surrounded by a thermoelectric heating system with a copper tube attached to the bottom, located below a microsyringe surrounded by a cooling system.

The crude oil sample is placed into the tube, which is then sealed with a silicone pad, while 20μL of an organic solvent such as hexane is added into the microsyringe. The needle of the microsyringe is then inserted through the silicone pad into the tube, which is heated to around 300°C, evaporating the volatile and semi-volatile compounds in the crude oil. A flow of nitrogen coming through the copper tube then drives the volatile compounds through the needle and into the organic solvent in the microsyringe, which is cooled to a temperature of -6°C. This cooling ensures that all the volatile compounds condense into the organic solvent.

Zhu and his team found that 10 minutes was sufficient for this method to extract all the volatile and semi-volatile compounds in the oil sample, including the diamondoids, into the organic solvent, which can then be analyzed directly by gas chromatography-mass spectrometry (GC-MS). The great advantage of GP-MSE is that the heated tube is kept physically separate from the cooled microsyringe, meaning that the volatile compounds can be efficiently evaporated without running the risk of evaporating the organic solvent or any of the extracted compounds.

Diamond in the rough

As a first test of GP-MSE, the scientists applied it to a specially-prepared solution of 27 different diamondoids, prior to analysis by GC-MS. Compared with direct injection of the solution or extraction by single-drop microextraction (SDME), the 27 diamondoids produced much higher peaks when extracted with GP-MSE. As a result, GP-MSE was between 78 and 102 times more sensitive than direct injection and nine to 16 times more sensitive than SDME. Next, Zhu and his team showed that GP-MSE was just as effective as column chromatography at extracting volatile compounds from crude oil, despite being much faster and using much less organic solvent.

Finally, they tried using GP-MSE to extract diamondoids from real-world samples of crude oil, prior to analysis by two-dimensional GC-MS. Using this approach, they were able to detect 100 different diamondoids in the crude oil samples, together with 27 other compounds, showing that diamondoids could be just the beginning for GP-MSE.

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